Flat miniature heat pipes are an emerging technology in the cooling of high heat flux electronic devices such as computer chips and thyristors. They have shown promise as an effective mechanism for distributing thermal load from semiconductor devices and reducing localized hot spots. But a severe limitation of these devices is that most of them are only capable of transferring heat axially. Recently, a new type of high performance heat spreader plates have been developed by Sandia National Laboratories. These structures can be fabricated as an integral part of large-scale silicon wafers and allow heat to be dissipated in any direction across the wafer, thereby vastly increasing the performance. Using photolithography wick features were generated on a thin Kovar sheet. A companion wafer was etched and the two pieces were wafer bonded to form a micro scale flat plate heat pipe.

Radial wick pattern etched on the substrate

Double layered wick design to optimize liquid flow

Liquid vaporizes near the center and the vapor condenses near the edges. The condensate returns to the center by the capillary action of the wick structures etched on the substrate. In order to further improve the heat removal capacity of such structures it is essential to understand the heat transfer process taking place and also the operating boiling limits. This project involves visualization of the heat transfer process on such structures using a high speed video camera equipped with a microscope attachment.

The structured surface was housed in a Plexiglas box and formed the evaporator section. A remote cold plate acted as the condenser and PFA tubing was used to connect the evaporator to the condenser. Experiments were conducted using  this set-up with FC-72 as the working fluid.  A thin thermofoil heater  (1Sq. inch)  was attached to the backside of the heat spreader plate and thermocouples(T-type, 5 mil) attached to various locations on the structure were used to sense temperature. Data was recorded using  Lab view software on a Hewlett Packard data acquisition system. By employing these thermocouples, degree of superheat and  heat flux measurements have been made. Pressure  was measured using a compound pressure gage (range: -30 in Hg to 350 Psi). Visualization of the heat transfer process on the structure was performed and the pictures recorded were analyzed.

Photograph of the experimental set-up 

Photograph of the evaporator section

Animation of bubble release at 4.75 W

The pictures were taken with a high speed video camera (500 frames/s with full frames) with a microscope attachment (10X-63X).

To simulate the actual functioning of the micro heat pipe more closely a second generation set-up is being proposed. Currently, I am designing the new set-up using PRO ENGINEER and subsequently it will be fabricated and experiments will be conducted using it.